Surface acoustic wave device

ABSTRACT

A surface acoustic wave device includes an asymmetrical double electrode which prevents a mismatch between reflected waves and propagating surface acoustic waves on strips, and which is capable of realizing a superior unidirectionality. This surface acoustic wave device includes the asymmetrical double electrode in which a half wavelength section includes first and second strips which have mutually different widths. The half wavelength is arranged to define a basic section. The surface acoustic wave device includes at least two of these basic sections disposed on a piezoelectric substrate. The absolute value of the vector angle of the reflection center is within approximately 45±10° or within approximately 135±10°, when the center of the-basic section is the reference position. Alternatively, the absolute value of the phase difference between the excitation center and the reflection center is within approximately 45±10° or approximately 135±10°.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave device for usein, for example, a resonator or a filter, and more particularly, to asurface acoustic wave device having an asymmetrical double electrodeused as a unidirectional interdigital transducer or a dispersivereflection type reflector.

2. Description of the Related Art

A surface acoustic wave device such as a surface acoustic wave filter iswidely used in mobile communication equipment or broadcasting equipment,or other such apparatuses. Particularly because the surface acousticwave device is compact, lightweight, tuning-free and easy tomanufacture, the surface acoustic wave device is suitable for anelectronic component for use in portable communication equipment.

The surface acoustic wave device is broadly divided into a transversaltype filter and a resonator-type filter, based on its structure. Ingeneral, the transversal type filter has advantages of having (1) asmall group delay deviation, (2) a superior phase linearity, and (3) ahigh degree of flexibility in the pass band design based on weighting.However, the transversal type filter has a disadvantage of having alarge insertion loss.

An interdigital transducer (hereinafter referred to as an “IDT”) used ina surface acoustic wave filter transmits and receives surface acousticwaves with respect to both sides of an IDT, that is, the IDT transmitsand receives surface acoustic waves bilaterally in an equal manner. Forexample, in a transversal type filter in which two IDTs are spaced apartfrom each other by a predetermined distance, one half of the surfaceacoustic waves transmitted from one IDT is received by the other IDT,but the surface acoustic waves propagated from the one IDT to theopposite side of the other IDT become a loss. This loss is called a“two-way loss”, and has become a big factor in increasing insertion lossof a transversal type filter.

In order to reduce the above-described two-way loss, various types ofunidirectional IDTs have been proposed. In such unidirectional IDTs,surface acoustic waves are transmitted and received at only one sidealone thereof. Also, low-loss transversal type filters which utilizethese unidirectional IDTs have been developed.

For example, Hanma et al., have proposed an asymmetrical doubleelectrode in “A TRIPLE TRANSIT SUPPRESSION TECHNIQUE” 1976 IEEEUltrasonics Symposium Proceedings pp. 328-331. FIG. 14 is a schematicpartially cutaway plan view showing the asymmetrical double electrodedisclosed in this prior art.

In an asymmetrical double electrode 101, half wavelength sections Zconstituted of two strips 102 and 103 having different widths from eachother, are disposed repeatedly many times along the propagationdirection of surface acoustic waves. Such an electrode defined by halfwavelength sections Z constituted of two strips having different widthsfrom each other, is called an “unbalanced double electrode” or a“asymmetrical double electrode”.

The width of a half wavelength section is set to 0.5λ. The width of astrip 102 having a relatively narrow width is set to λ/16. The width ofa strip 103 having a relatively wide width is set to 3λ/16. The width ofa gap between the strips 102 and 103 is set to 2λ/16. The width of anouter gap of the strip 102 in the half wavelength section is set toλ/16. The width of the outer gap of the strip 103 in the propagationdirection of surface acoustic waves in the half wavelength section isset to λ/16.

Between adjacent basic sections, the electrical polarities are oppositeto each other.

In the above-described asymmetrical double electrode, a reflection perbasic section can be expressed by a resultant vector that is generatedby synthesizing reflected waves from the edges X1 to X4 of the strips102 and 103 shown in FIG. 15. FIG. 16 shows the reflection vectors atthe edges X1 to X4 when the reference position is set to the center of abasic section, and the resultant vector thereof. As can be seen fromFIG. 16, the resultant vector V is located at an angle of 67.5°, and thereflection center is located at an angle of 67.5°/2=33.75°.

Also, in this asymmetrical double electrode, the outer edge X1 of thestrip 102 and the outer edge X4 of the strip 103 are disposedbilaterally symmetrically with respect to the center of the halfwavelength section. Hence, the distances between the center of a basicsection and the outer edges of the nearest strips in the adjacent basicsections, are also equal to each other. In the asymmetrical doubleelectrode, therefore, an excitation center is located at the center ofthe basic section Z, with a phase difference of about 33.750 generatedbetween an excitation center and the reflection center. Thus, theasymmetrical double electrode operates as a unidirectional electrode.

Table 1 below shows the inter-mode coupling coefficient κ₁₂/k₀, thephase difference between the excitation center ψ and the reflectioncenter φ, and the reflection center φ, when forming an asymmetricaldouble electrode of aluminum film having a 3% film-thickness on a ST-cutcrystal quartz substrate, as an example of the above-describedasymmetrical double electrode. TABLE 1 Item Calculated value Inter-modecoupling coefficient κ₁₂/k₀  0.00257 Phase difference between excitationcenter ψ 31.3° and reflection center φ Reflection center φ 33.8°

Here, k₀ is a wave number of surface acoustic waves propagating throughan IDT. The ratio κ₁₂/k₀ and the phase difference between the excitationcenter ψ and the reflection center φ can be obtained from the resonantfrequency determined by the finite element method, using the techniqueof Cbuchi et al., (“Evaluation of Excitation Characteristics of SurfaceAcoustic Wave Interdigital Electrode Based on Mode Coupling Theory”,Institute of Electronics, Information and Communication Engineers ofJapan, Technical Report MW90-62). Also, the reflection center φ isdetermined by the phase difference between the excitation center ψ andthe reflection center φ, and the excitation center obtained from thefundamental wave component which is acquired by Fourier-transforming theelectric charge density distribution on the electrode obtained by thefinite element method.

Japanese Unexamined Patent Application Publication No. 61-6917 disclosesan electrode which has implemented unidirectionality by disposing twostrips having mutually different widths in a half wavelength section, asin the case of the above-described asymmetrical double electrode. Theelectrode disclosed in this Japanese Unexamined Patent ApplicationPublication No. 61-6917 is also supposed to operate as a unidirectionalelectrode due to the asymmetry of the two strips thereof. However, inthe method disclosed in the Japanese Unexamined Patent ApplicationPublication No. 61-6917, no means for controlling the reflection centerand the reflection amount are disclosed. In addition, no feasiblereflection center and reflection amount are described.

The article “Direct Numeral Analysis SAW Mode Coupling Equation andApplications Thereof”, 27th EM symposium preprint, pp. 109-116, Takeuchiet al., describes the principle of a unidirectional IDT which providesflat directivity over a wide band in the structure wherein positive andnegative reflection elements are dispersively disposed in aunidirectional IDT. Herein, however, no means for forming a reliablysuperior unidirectional IDT are described.

In general, when surface acoustic waves are caused to be incident on anIDT constituted only of double strips without reflection, reflection iscaused by re-excitation. As a result, in the case of a conventionaltransversal type filter, waves called “triple transit echo” or TTE,occur, and cause ripples or other undesired wave characteristics thatadversely effect filter characteristics. The above-described literatureby Hanma et al., discloses a method for canceling out reflection due tore-excitation by means of acoustic reflected waves of an asymmetricaldouble electrode. This method, however, has created a problem that newripples are caused by acoustic reflection when the acoustic reflectionis larger than the reflection caused by the re-excitation. Therefore,such a method for canceling out the reflection by re-excitation issubjected to the restriction of piezoelectric substrate material orelectrode material, since the reflection vector length which representsthe acoustic reflection amount is fixed in an asymmetrical doubleelectrode.

On the other hand, the article “About One Weighting Method For SAWReflector”, 1999, General Convention of Institute of Electronics,Information and Communication Engineers of Japan, p. 279, Tajima et al.,discloses a method for performing weighting with respect to thereflection coefficient of a reflector. This method uses a plurality ofstrips having mutually different widths and makes use of the change ofthe reflection coefficient of a strip based on the strip width. However,when the strip width is changed, the sonic speed is also changed. As aresult, when attempting to perform weighting based on the strip width, atesting method and apparatus is needed to find a correct sonic speed andto change the arrangement pitch of the strip in accordance with thiscorrected sonic speed. This poses a problem that the design requires anextremely high degree of technique.

As described above, various IDTs or resonators each operating as aunidirectional electrode by asymmetry of two strips have been proposed,but conventional asymmetrical double electrodes have not yet achievedsufficient unidirectionality. In addition, the reflection center and thereflection amount of the conventional asymmetrical double electrodeshave been very difficult to control.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a surface acoustic wave device using anasymmetrical double electrode which achieves more superiorunidirectionality of surface acoustic wave propagation while effectivelyand easily controlling the reflection amount per basic section.

In accordance with a preferred embodiment of the present invention, asurface acoustic wave device includes a piezoelectric substrate, andincluding at least two basic sections including an asymmetrical doubleelectrode in which a half wavelength section includes first and secondstrips having different widths from each other, the at least two basicsections being disposed along the propagation direction of surfaceacoustic waves. In this surface acoustic wave device, the absolute valueof the vector angle of the reflection center obtained from the resultantvector generated by synthesizing the reflection vectors at the edges ofthe first and second strips is preferably within approximately 45±10° orapproximately 135±10°, when the center of the each of the at least twobasic sections is the reference position.

In accordance with another preferred embodiment of the presentinvention, a surface acoustic wave device includes a piezoelectricsubstrate, and including at least two basic sections including anasymmetrical double electrode in which a half wavelength sectionincludes first and second strips having different widths from eachother, the at least two basic sections being disposed along thepropagation direction of surface acoustic waves. In this surfaceacoustic wave device, the absolute value of the phase difference betweenthe excitation center and the reflection center of the asymmetricaldouble electrode, is preferably within approximately 45±10° orapproximately 135±10°.

In accordance with a still another preferred embodiment of the presentinvention, a surface acoustic wave device includes a piezoelectricsubstrate, and including at least two basic sections including anasymmetrical double electrode in which a half wavelength sectionincludes first and second strips having different widths from eachother, the at least two basic sections being disposed along thepropagation direction of surface acoustic waves. In this surfaceacoustic wave device, when the edge positions of the first and secondstrips are X1 to X4, each of which is a value corrected using the sonicspeed difference between a free surface and a metallic surface, and whenthe resultant vector length of normalized reflected waves from the stripedges is |Γ|, and the center position of the basic section is 0(λ), andX1≅X4, each of the positions of X2 and X3 is a value substantiallysatisfying the following equations (1) and (2).

Mathematical Expression 4X 2 [λ]=A×X 1[λ]² +B×X 1[λ]+C±0.1[λ]  (1)Mathematical Expression 5X 3 [λ]=D×X 1[λ]² +E×X 1[λ]+F±0.05[λ]  (2)Mathematical Expression 6A=−34.546×|Γ|⁶+176.36×|Γ|⁵−354.19×|Γ|⁴+354.94×⊕Γ|³−160.44×|Γ|²+10.095×|Γ|−1.7558B=−15.464×|Γ|⁶+77.741×|Γ|⁵−153.44×|Γ|⁴+147.20×|Γ|³−68.363×|Γ|²+6.3925×|Γ|−1.7498C=−1.772×|Γ|⁶+8.7879×|Γ|⁵−17.07×|Γ|⁴+16.092×|Γ|³−7.4655×|Γ|²+0.8379×|Γ|−0.3318D=12.064×|Γ|⁶−45.501×|Γ|⁵+57.344×|Γ|⁴−22.683×|Γ|³+12.933×|Γ|²−15.938×|Γ|−0.1815E=7.2106×|Γ|⁶−30.023×|Γ|⁵+45.792×|Γ|⁴−29.784×|Γ|³+13.125×|Γ|²−6.3973×|Γ|+1.0203F=1.0138×|Γ|⁶−4.4422×|Γ|⁵+7.3402×|Γ|⁴−5.474×|Γ|³+2.3366×|Γ|²−0.7540×|Γ|+0.2637

In the surface acoustic wave device in accordance with another preferredembodiment of the present invention, it is preferable that thereflection amounts of the surface acoustic waves at the edge positionsX1 to X4 of the above-described strips be substantially equal to oneanother.

Also, in the surface acoustic wave device in accordance with otherpreferred embodiments of the present invention, the above-describedasymmetrical double electrode may be an interdigital transducer, or mayinstead be a reflector.

Furthermore, in accordance with another preferred embodiment of thepresent invention, preferably, quartz crystal is preferably used as theabove-described piezoelectric substrate. Alternatively, however, inother preferred embodiments of the present invention, the piezoelectricsubstrate may be constituted of another piezoelectric single crystalsuch as LiTaO₃, or a piezoelectric ceramic such as lead titanatezirconate-based ceramic. Also, a piezoelectric substrate constructed byforming a piezoelectric thin-film such as a ZnO thin-film on aninsulative substrate such as a piezoelectric substrate or aluminasubstrate, may be used.

The above and other elements, characteristics, features, and advantagesof the present invention will be clear from the following detaileddescription of preferred embodiments of the present invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of an asymmetrical double electrode in accordancewith a preferred embodiment of the present invention;

FIG. 1B is a partially cutaway sectional view of an asymmetrical doubleelectrode in accordance with a preferred embodiment of the presentinvention;

FIG. 2 is a diagram showing the edge-position dependence of theexcitation center of the asymmetrical double electrode in a preferredembodiment of the present invention;

FIG. 3 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 0.20λ.

FIG. 4 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 0.50λ.

FIG. 5 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 1.00λ.

FIG. 6 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 1.25λ.

FIG. 7 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 1.50λ.

FIG. 8 is a diagram showing the relationship between the edge positionX1=−X4 and each of the edge positions X2 and X3, when a resultant vectorlength Γ is 1.70λ.

FIG. 9 is a diagram showing the change in the reflection center φ whenthe edge position X2 obtained by the equation (1) changes, in preferredembodiments of the present invention.

FIG. 10 is a diagram showing the change in the reflection center φ whenthe edge position X3 changes in preferred embodiments of the presentinvention.

FIG. 11 is a schematic plan view showing the electrode structure, forevaluating directivity of an IDT in accordance with another preferredembodiment of the present invention.

FIG. 12 is a diagram showing the relationship between the number ofbasic sections and the directivity, which relationship has been obtainedin a further preferred embodiment of the present invention, and therelationship between the number of the basic sections and thedirectivity when using a conventional asymmetrical double electrodeprepared for comparison.

FIG. 13 is an explanatory plan view of the electrode structure of an IDThaving a reflector in accordance with yet another preferred embodimentof the present invention.

FIG. 14 is a schematic partially cutaway plan view showing aconventional asymmetrical double electrode.

FIG. 15 is a partially cutaway sectional view for explaining the edgepositions of the strips in the asymmetrical double electrode shown inFIG. 14.

FIG. 16 is a diagram showing the relationship between the reflectionvectors in the edges X1 to X4 shown in FIG. 15 and the resultant vectorV thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to realize the unidirectionality using an asymmetrical doubleelectrode, the inventors of the present application have conductedextensive research and have discovered that, when the reflection amountof surface acoustic waves per basic section is small, the frequencyunidirectionality characteristics of the unidirectional electrode can beestimated by forming reflection elements using a unidirectionalelectrode wherein the phase difference between the excitation center andthe reflection center is approximately +45° (−135°) or approximately−45° (+135°), and by disposing these positive and negative reflectionelements, regarding them as positive and negative impulses,respectively. Furthermore, the present inventors have discovered that,when the phase difference between the excitation center and thereflection center largely deviates from approximately ±45° (±135°) inthe positive and negative elements, it becomes difficult to regard asthe positive and negative elements as simple positive and negativeimpulses, respectively, because of the phase mismatching of surfaceacoustic waves.

Moreover, the present inventors have discovered that, when a weightingmethod in a unidirectional IDT using an asymmetrical double electrode isused, it is possible to perform weighting with respect to reflectioncoefficients, when positive and negative reflection elements wherein thereflection centers thereof are located at angles of approximately +45°(−135°) and −45° (+1350), respectively, with respect to the center of ahalf wavelength section, are formed and are utilized as a reflector.When attempting to perform weighting to the strip width, it has beennecessary to change the electrode pitch. However, this weighting methodby reflection coefficient allows a reflector to be easily produced,since sonic speeds of the positive and negative elements are identicalwith each other.

Next, the principles of various preferred embodiments of the presentinvention will be described in more detail with reference to thedrawings.

An asymmetrical double electrode 1 shown in FIGS. 1A and 1B is taken asan example. In this asymmetrical double electrode 1, basic sections Zeach of which is constituted of strips 2 and 3 having mutually differentwidths, are repeatedly arranged in the propagation direction of surfaceacoustic waves. Now, let one basic section be disposed at the positionsfrom −0.25λ to +0.25λ. Here, λ denotes the wavelength of a surfaceacoustic wave.

Letting the positions of the edges of the first and second strips 2 and3 be disposed within this basic section, that is, this half wavelengthsection be X1′ to X4′, and the sonic speed of surface acoustic wavespropagating through a free surface be V_(f), and the sonic speed ofsurface acoustic waves propagating through a metallic surface be V_(m),the edge positions X1 to X4 corrected based on the sonic speeds of thefree surface and the metallic surface are expressed by the followingequation:

Mathematical Expression 7X 1 to X 4=(V _(f) L _(m) +V _(m) L _(f))/(V _(f) L _(m0) +V_(m)L_(f0))  (3)

In the above equation (3), L_(m) denotes the sum of the distance on themetallic surface from the center of the half wavelength section, thatis, 0λ to X1 to X4 in the propagation direction of surface acousticwaves, and L_(f) denotes the sum of the distance on the free surfacefrom the center of the half wavelength section, 0λ to X1 to X4. L_(m0)denotes the sum of the distance of the metallic surface in the entirehalf wavelength section, and L_(f0) denotes the sum of the distance ofthe free surface in the entire half wavelength section.

Next, the reflection in a single electrode in which only a single stripis disposed within the half wavelength section, will be discussed.Suppose that the single strip is arranged so that the center thereof islocated at the reference position 0λ of the half wavelength section Z.Letting the reflection vector at the one edge position ΓXs of the singlestrip be Γs1, and the reflection vector at the other edge position +Xsthereof be Γs2, the resultant reflection vector Γs at the referenceposition is expressed by the equation (4) below. Here, j in the equation(4) denotes an imaginary number, and k denotes the wave number.

Mathematical Expression 8Γs=Γs 1×e ^(−2·j·k·(−Xs)) +Γs 2×e ^(−2·j·k·Xs)  (4)

The length |Γs| of the above-described resultant vector Γs denotes thereflection amount of a single strip.

Here, when conducting a normalization such as |Γs1|=|Γs2|=1, we canexpress Γs1=−Γs2=−1 under the condition that the acoustic impedance on afree surface is larger than that on a metallic surface. Therefore, whendefining the reflection center φs as the center of the single strip, thereflection center φs can be determined by the following equation (S),using the angle ∠ F of the resultant reflection vector Γ.

Mathematical Expression 9φs=−0.5×∠(j×Γs)  (5)

Next, discussion will be made of an asymmetrical double electrodewherein two strips having mutually different widths are disposed in thehalf wavelength section, as in the case of the single strip. Letting thereflection vectors of surface acoustic waves at the edge positions X1 toX4 in FIGS. 1A and 1B be Γ1 to Γ4, the resultant reflection vector Γ atthe reference position 0λ is expressed by the equation (6) below.

Mathematical Expression 10Γ=Γ1×e ^(−2·j·k·X1)+Γ2×e ^(−2·j·k·X2)+Γ3×e ^(−2·j·k·X3) +Γ4×e^(−2·j·k·X4)  (6)

The length |Γ| of the above-described resultant vector Γ denotes thereflection amount of a unidirectional electrode. The reflection centerof the unidirectional electrode is defined in the same way as the singlestrip, and is expressed by the equation (7) below.

Mathematical Expression 11φ+=−0.5×∠(j×Γ)  (7)

In the case where, in the asymmetrical double electrode, aunidirectional IDT is constructed such that the electric polarities ofadjacent basic sections are alternately inverted, when the width of theinter-strip gap between a basic section and the adjacent basic sectionon one side in the propagation direction of surface acoustic waves, andthe width of the inter-strip gap between the basic section and theadjacent basic section on the other side in the propagation direction ofsurface acoustic waves, are equal to each other, and simultaneously whenthese inter-strip gaps are disposed symmetrically with respect to thecenter of the center basic section, the excitation center of theasymmetrical double electrode is located at the substantially centralportion of the half wavelength section.

FIG. 2 is a diagram showing the edge-position dependence of theexcitation center in the above-described asymmetrical double electrode.Herein, an asymmetrical double electrode formed of an aluminum filmhaving a thickness of, for example, approximately 0.02λ, is disposed ona ST-cut quartz substrate. In this figure, there is shown the edgeposition dependence of the excitation center obtained from thefundamental wave component which is acquired by Fourier-transforming theelectric charge density distribution on the electrode obtained by thefinite element method, when X1=−X4=−0.1875λ, and X3−X2=0.125λ, and whenX2 is used as a parameter.

It can be confirmed that even at a position wherein the degree ofasymmetry of the asymmetrical double electrode is very high, that is, atX2=0.172λ, the vector angle of the excitation center is located at about+4.6°, that is, substantially at the central portion. The strip widthand the gap width of an IDT constituting a surface acoustic wave deviceis restricted by the electrical resistance of a strip and/or thepatterning process.

The edge positions X and X3 can be uniquely determined with respect tothe |Γ| and the edge position X1, by letting X2−X1>0.02λ, X3−X2>0.02λ,X4−X3>0.02λ, and X4=−1, assuming that the vector lengths of Γ1 to Γ4 areequal to one another, performing a normalization such that Γ1=Γ4=−1,Γ2=Γ3=+1, and finding the conditions such that the equations (6) and (7)satisfies φ=45°, by the Monte Carlo method. The approximate equationsexpressing X2 and X3 are given by the following expressions (8) and (9),using |Γ| and X1 as independent variables.

Mathematical Expression 12X 2[λ]≅A×X 1[λ]² +B×X 1[λ]+C  (8)Mathematical Expression 13X 3[λ]≅D×X 1[λ]² +E×X 1[λ]+F  (9)

In the equations (8) and (9), A to F are obtained by the followingequations.

Mathematical Expression 14A=−34.546×|Γ|⁶+176.36×|Γ|⁵−354.19×|Γ|⁴+354.94×⊕Γ|³−160.44×|Γ|²+10.095×|Γ|−1.7558B=−15.464×|Γ|⁶+77.741×|Γ|⁵−153.44×|Γ|⁴+147.20×|Γ|³−68.363×|Γ|²+6.3925×|Γ|−1.7498C=−1.772×|Γ|⁶+8.7879×|Γ|⁵−17.07×|Γ|⁴+16.092×|Γ|³−7.4655×|Γ|²+0.8379×|Γ|−0.3318D=12.064×|Γ|⁶−45.501×|Γ|⁵+57.344×|Γ|⁴−22.683×|Γ|³+12.933×|Γ|²−15.938×|Γ|−0.1815E=7.2106×|Γ|⁶−30.023×|Γ|⁵+45.792×|Γ|⁴−29.784×|Γ|³+13.125×|Γ|²−6.3973×|Γ|+1.0203F=1.0138×|Γ|⁶−4.4422×|Γ|⁵+7.3402×|Γ|⁴−5.474×|Γ|³+2.3366×|Γ|²−0.7540×|Γ|+0.2637

From the above results, it can be recognized that an asymmetrical doubleelectrode which corresponds to a desired reflection amount, and havingthe reflection center at an angle of about 45° can be obtained. As canfurther be recognized, in an asymmetrical double electrode which isconstructed in accordance with the equations described above, theexcitation center is located at the center of a half wavelength section.As a result, when this asymmetrical double electrode is used as aunidirectional electrode, the phase difference between the excitationand the reflection center becomes substantially 45°, allowing thisasymmetrical double electrode to operate as a unidirectional electrodehaving very superior characteristics.

As examples, FIGS. 3 to 8 show the results of X2 and X3 obtained byequations (8) and (9), for |Γ|=0.20λ, 0.50λ, 1.00λ, 1.25λ, 1.50λ, and1.70λ. Meanwhile, in the above description, the reflection coefficienthas been treated based on the premise that the acoustic impedance on afree surface is larger that that on a metallic surface. Conversely,under the condition that the acoustic impedance on a free surface issmaller that that on a metallic surface, only the sign of |Γ| isreversed, or in other words, that φ is shifted by 90°.

As described above, by selecting the edge positions X2 and X3 so as tosatisfy the equations (8) and (9), the phase difference between theexcitation center and the reflection center can be made substantially45°. As a result, a very superior unidirectional electrode can beachieved. However, the present inventors have confirmed that thisasymmetrical double electrode has a very excellent unidirectionality, ifX2 and X3 are located not only at the positions satisfying the equations(8) and (9), but also at the positions within a certain range from thepositions satisfying the equations (8) and (9). This fact will bedescribed with reference to FIGS. 9 and 10.

FIGS. 9 and 10 are diagrams each showing the changes in the reflectioncenter when X2 and X3, each obtained by substituting |Γ|=1.5 andX1=−0.2188λ into the equations (8) and (9), within the range from −0.1λto +0.1λ.

As described above, it is desirable that the reflection center belocated at an angle of approximately 45°, or the phase differencebetween the reflection center and the excitation center be approximately45°, but the present inventors have confirmed that the range withinapproximately 45±10° would allow the phase mismatching to be greatlyimproved as compared to the above-described prior art asymmetricaldouble electrode. It can be seen from FIGS. 9 and 10 that the range suchthat the position of the reflection center is at an angle ofapproximately 45±10°, corresponds to the range of about ±0.10λ withrespect to the value obtained by the equation (8) for the position ofX2, and corresponds to the range of about ±0.05λ with respect to thevalue obtained by the equation (9) for the position of X3.

In preferred embodiments of the present invention, therefore, thepositions of X2 and X3 are preferably within the range shown in theabove-described equations (1) and (2). It will be understood that asuperior unidirectionality can be realized as a result of this uniquearrangement.

A surface acoustic wave device using an asymmetrical double electrode inaccordance with preferred embodiments of the present invention wasconstructed as illustrated in FIG. 1. An IDT was constructed by formingan aluminum film having a thickness of, for example, approximately 0.02λon a ST-cut quartz substrate, and then performing patterning.

The IDT defining an asymmetrical double electrode was constructed inaccordance with the edge positions X2 and X3 which were determined bysubstituting the values of |Γ| and X1 shown in Table 2 below into theequations (8) and (9). Table 2 shows the inter-mode couplingcoefficients κ₁₂/k₀ and the reflection centers φ in this case.

In the asymmetrical double electrode, shown in FIG. 2, which isconstructed based on the equations (8) and (9), since the angle of thereflection center is close to 45°, the phase mismatching of thereflected waves with respect to the propagating waves is significantlyless than that of the conventional asymmetrical double electrode.Therefore, the use of the asymmetrical double electrode constructedbased on the equations (8) and (9), allows a surface acoustic wavedevice which performs much better than the conventional surface acousticwave devices to be achieved, and is particularly effective whenpositively making use of the reflection of strips. TABLE 2 Reflection|Γ| X1 [λ] κ₁₂/k₀ center φ [λ] 0.20 −0.19 0.0005 43.8 0.50 −0.20 0.001542.3 0.75 −0.20 0.0021 40.5 1.00 −0.21 0.0029 39.6 1.25 −0.21 0.003139.0 1.50 −0.22 0.0036 39.2 1.60 −0.22 0.0035 40.2 1.70 −0.23 0.003841.5 1.73 −0.23 0.0038 42.2

Next, description will be made of specific experimental examples of thedirectivity when an IDT including an asymmetrical double electrode isprovided on a ST-cut quartz substrate, in accordance with a preferredembodiment of the present invention.

As shown in FIG. 11, IDT 11, IDT 12, and IDT 13 were formed on a ST-cutquartz substrate (not shown) using an aluminum film having a thicknessof, for example, approximately 0.02λ. The middle IDT 11 is constitutedof an asymmetrical double electrode in accordance with preferredembodiments of the present invention, and IDT 12 and IDT 13 disposed onthe opposite sides of IDT 11 are ordinary double electrode type IDTs.

In IDT 11 constituted of an asymmetrical double electrode, when the edgeportions of the first and second strips 2 and 3 having different widthsare made asymmetric, the excitation center deviates from the center ofthe half wavelength section, so that the phase difference between theexcitation and the reflection center also deviates from approximately45°. Therefore, the edge positions X2 and X3 obtained by substituting|Γ|=1.5, and X1=−0.2188λ into the equations (8) and (9), were adjustedby about 0.05λ and corrected so that the phase difference between theexcitation and the reflection center approaches approximately 45°.

As a result, when X1=−0.2188λ, X2=−0.1185λ, X3=+0.0050λ, andX4=+0.2188λ, the phase difference between the excitation center and thereflection center became about 41°.

FIG. 12 shows the comparison between the directivity of IDT 11 whichuses the electrode structure shown in FIG. 11 and which includes theasymmetrical double electrode having the above-described construction,and the directivity when the conventional asymmetrical double electrodeis disposed in place of IDT 11. The solid line in the figure shows theresult of IDT 11, and the broken line shows that of the conventionalexample. With regard to the directivity, an input voltage is applied toIDT 11, then the output thereof received by IDT 12 and IDT 13 wassought, and the directivity was evaluated from the value of this output(dB).

For an IDT using an asymmetrical double electrode prepared forcomparison, the film thickness of the electrode was preferably set toabout 0.02λ, and the edge positions were preferably set so as to beX1=−0.1875λ, X2=−0.1250λ, X3=0λ, and X4=+0.1875λ. The crossing width ofan electrode finger was preferably set to about 20λ in each of thepreferred embodiments and the conventional example.

For IDT 12 and IDT 13 on the opposite sides of IDT 11, the crossingwidth of an electrode finger were preferably set to about 20λ, and theedge positions were preferably set so as to be X1=−0.1875λ, X2=−0.0625λ,X3=+0.0625λ, and X4=+0.1875λ.

It can be recognized from FIG. 12 that the asymmetrical double electrodein this preferred embodiment has a better unidirectionality than that ofthe conventional asymmetrical double electrode. In addition, the presentinventors have confirmed that the phase difference between theexcitation center and the reflection center can be corrected so as toapproach 45° by adjusting the edge positions X2 and X3 obtained by theequations (8) and (9) by about ±0.1λ, or by adjusting X4 so as toslightly depart from −X1.

FIG. 13 is a plan view showing the electrode structure of an IDT havingan reflector 21 according to yet another preferred embodiment of thepresent invention. Herein, the reflector 21 constructed in accordancewith this preferred embodiment of the present invention is preferablydisposed within IDT 22. In this case, by performing weighting withrespect to the reflection coefficient of the reflector 21, it ispossible to control the frequency characteristics of the entire IDT 22having the reflector 21.

The present invention is not limited to the above-described preferredembodiments, but can be variously modified. For example, in theabove-described preferred embodiments, it is recognized that a betterdirectivity than that of the conventional example is achieved. However,there may be a case, depending on the use, where it is more importantthat the phase difference between the excitation center and thereflection center is close to 45°, or that the reflection center whenX1=−X4, is 45° with respect to the center of the half wavelengthsection, rather than achieving better directivity. Although it isdesirable that the phase difference between the excitation center andthe reflection center be about 45°, there may be a case where, when thereflection by a strip is positively utilized, for example, when it isused as a reflector, priority is given to the feature that thereflection center is located at an angle of 45°, over the feature thatthe excitation center is located at the center of the half wavelengthsection, even if the excitation center deviates therefrom. Particularlywhen a strip is utilized as a reflector, only the reflection center canbe taken into consideration.

As is evident from the foregoing, in the surface acoustic wave deviceusing an asymmetrical double electrode in accordance with variouspreferred embodiments of the present invention, the absolute value ofthe vector angle of the reflection center obtained from the resultantvector formed by synthesizing the reflection vectors at the edges X1 toX4 of the first and second strips when the center of the above-describedbasic section is set to be the reference position, is preferably withinapproximately 45±10° or approximately 135±10°. Thereby, the phasemismatching of surface acoustic waves is minimized, and theunidirectionality of the above-described asymmetrical double electrodeis greatly improved.

Likewise, in various preferred embodiments of the present invention,when the absolute value of the phase difference between the excitationcenter and the reflection center of the asymmetrical double electrode,is within approximately 45±10° or approximately 135±10°, the phasemismatching of surface acoustic waves is minimized, and superiorunidirectionality can be realized.

In preferred embodiments of the present invention, in the edge positionsX1 to X4 of the first and second strips, which constitute basic sectionsand which have mutually different widths, when the center position ofthe basic section is 0(λ), and X1≅−X4, if the positions of X2 and X3satisfy the equations (1) and (2), it is ensured that the absolute valueof the vector angle of the reflection center is within approximately45±10° or approximately 135±10° when the center of the basic section isthe reference position, or that the absolute value of the phasedifference between the excitation center and the reflection center iswithin approximately 45±10° or approximately 135±10°. It is, therefore,possible to easily and reliably provide, in accordance with preferredembodiments of the present invention, an asymmetrical double electrodewhich prevents the phase mismatching of surface acoustic waves, andwhich has a superior unidirectionality.

When the reflection amounts of surface acoustic waves at the edgepositions X1 to X4 are substantially equal to one another, the phasemismatching between reflected surface acoustic waves and propagatingsurface acoustic waves is very effectively reduced.

When an IDT is constructed to include the asymmetrical double electrode,in accordance with various preferred embodiments of the presentinvention, the phase mismatching between reflected surface acousticwaves and propagating surface acoustic waves is prevented, therebyallowing an IDT having a superior unidirectionality to be provided, andenabling, for example, a low-loss transversal type surface acoustic wavedevice to be provided.

When the asymmetrical double electrode in accordance with preferredembodiments of the present invention is used as a reflector, sinceweighting can be easily performed with respect to the reflectioncoefficient, it is possible to provide a surface acoustic wave devicewhich is capable of controlling the frequency characteristics of theoverall reflection coefficient of reflectors.

While the present invention has been described with reference to whatare at present considered to be preferred embodiments, it is to beunderstood that various changes and modifications may be made theretowithout departing from the invention in its broader aspects andtherefore, it is intended that the appended claims cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

1-5. (canceled)
 6. A surface acoustic wave device, comprising: apiezoelectric substrate; and at least two basic sections disposed onsaid piezoelectric substrate, each of the at least two basic sectionsincluding an asymmetrical double electrode defining a half wavelengthsection and having first and second strips with different widths fromeach other; wherein an absolute value of a phase difference between anexcitation center and an reflection center of said asymmetrical doubleelectrode is within approximately 45+10° or approximately 135±10°.
 7. Asurface acoustic wave device according to claim 6, wherein reflectionamounts of surface acoustic waves at edge positions of said strips aresubstantially equal to one another.
 8. A surface acoustic wave deviceaccording to claim 6, wherein said asymmetrical double electrode is aninterdigital transducer.
 9. A surface acoustic wave device according toclaim 6, wherein said asymmetrical double electrode is a reflector. 10.A surface acoustic wave device according to claim 6, wherein saidpiezoelectric substrate is made of a quartz crystal material. 11-15.(canceled)